The present invention is directed to a positive electrode active material for a magnesium secondary battery and a magnesium battery with a cathode based on the active material.
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/eq and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Production of a battery having an anode based on magnesium requires a cathode which can reversibly adsorb and desorb magnesium ions and an electrolyte system which will efficiently transport magnesium ions. Significant effort in each of these areas is ongoing in many research organizations throughout the world and active materials under investigation include sulfur in various forms, including elemental sulfur, materials known as Chevrel compounds of formula MgxMo6Tn, (wherein x is a number from 0 to 4, T is sulfur, selenium or tellurium, and n is 8) and various metal oxides such as MnO2 (alpha manganese dioxide stabilized by potassium), V2O5 and ion stabilized oxide or hollandiates of manganese, titanium or vanadium.
In this regard, V2O5 is an extremely promising candidate for the Mg battery cathode, because it is capable of multiple redox reactions between V5+/V4+/V3+ and V metal. Also, V5+ as a high valence state is quite stable, which means that it is easy to increase the operating voltage. Various research groups have reported efforts directed to utility of V2O5 as a positive electrode active material.
Sakurai et al. (U.S. Pat. No. 4,675,260) describes an amorphous V2O5 prepared by adding at least one first additive selected from the group P2O5, TeO2, GeO2, Sb2O3, Bi2O3 and B2O3 and/or at least one second additive selected from MoO3 and WO3. A molten mixture of the components is prepared and then quenched. In a preferred embodiment the quenching is accomplished by passage through a twin roll quenching apparatus. The amorphous V2O5 is used as an active cathode material for a lithium battery. Both cathode construction and lithium battery construction and performance is described.
Tobishima et al. (U.S. Pat. No. 4,737,424) describes a lithium secondary battery containing a cathode having an amorphous V2O5 active material. Substantially pure V2O5 or V2O5 mixed with P2O5, TeO2, GeO2, Sb2O3, Bi2O3, GeO2, B2O3, MoO3, WO3 and TiO2 is indicated as a useful cathode active material. The amorphous material is prepared by melting a mixture of the components and then quenching the melt. Lithium secondary batteries with a cathode containing the amorphous V2O5 are described.
Noguchi et al. (U.S. Pat. No. 5,273,848) describes a cathode active material containing an amorphous solid solution of V2O5, P2O5 and an alkaline earth metal oxide (MO) and optionally CoO2. The amorphous material is prepared by rapidly quenching a melt of the components and in one embodiment the quench is conducted using twin copper rollers. Lithium batteries based on a cathode of the amorphous V2O5 mixture are described.
Kelley et al. (U.S. 2005/0079418) describes a method to prepare thin film batteries, including lithium, lithium ion and lithium free batteries. Materials described as useful as a cathode active material include amorphous V2O5. No actual working examples of batteries are provided and no description of a lithium free battery is provided.
Chen et al. (U.S. 2011/0070500) describes an electrode material prepared by combining an amorphous metal oxide and a crystalline metal oxide. The composite is then used in construction of an electrode. An example based on vanadium pentoxide is described as well as utility as a cathode for a lithium secondary battery.
Aoyagi et al. (U.S. 2012/0164537) describes a cathode material containing V2O5 crystallites within an amorphous phase of a combination of metal oxides. The amorphous phase metal oxide include vanadium, iron, manganese, silver, copper, cobalt, nickel, tungsten and boron. The crystallite/amorphous dual phase material is obtained by combining the metals as oxides and heating the mixture in an electric furnace to a temperature of approximately 900 to 1100° C. and then pouring the melt onto a stainless steel plate. A magnesium battery containing the dual phase material as a cathode active material is described.
Imamura et al. (Mg Intercalation Properties into V2O5 gel/Carbon Composites under High-Rate Condition; Journal of the Electrochemical Society, 150(6) A753-A758 (2003)) describes a V2O5 carbon composite material which when constructed into an electrode intercalates Mg ion. The composite is formed based on a V2O5 sol., i.e., a hydrated V2O5 crystal.
Miyayama et al. (Characterization of magnesium-intercalated V2O5/carbon composites; Solid State Ionics, 161(2003) 173-180) describes V2O5/carbon composites and studies Mg2+ reversible diffusion into the V2O5 xerogel structure. A structural model of the xerogel is described.
Amatucci et al. (Investigation of Yttrium and Polyvalent Ion Intercalation into Nanocrystalline Vanadium Oxide; Journal of the Electrochemical Society, 148 (8) A940-A950 (2001) describes studies showing that nanocrystalline V2O5 is capable of reversible intercalation of Mg2+. This reference provides description of the utility of nanocrystalline V2O5 as a cathode active material for univalent and multivalent ions.
Doe et al. (WO 2011/150093) describes a series of compounds which are suitable as cathode materials for a Mg battery. In one embodiment a V2O5 structure is prepared by first synthesizing a compound MgV2O5 and then removing the Mg by an electrochemical method. The resulting material is described as having a different stacking of V2O5 layers from directly prepared V2O5. Utility of the thus prepared differently stacked V2O5 as a cathode active material for a Mg battery is proposed.
However, each of the various forms of V2O5 described above fail to meet all the requirements necessary to function as a positive electrode active material in a magnesium battery which would supply the energy demands of a commercial electric vehicle. Nanocrystalline materials are difficult to be loaded densely in the cathode, which means that the thickness of the nanocrystalline-based cathode should be much thicker than that of micron-sized ones. That is to say that the volumetric energy density for a total cell based on such materials would be decreased. Also, nanocrystalline materials will promote electrolyte decomposition due to the extremely high surface area associated with such structure.
In hydrated forms of V2O5, much water acts as a ligand in the structure. During magnesium insertion/extraction in a typical non-aqueous media, water is an undesired molecule because a non-conducting and resistive blocking layer due to releasing water is formed on the counter anode.
The predicted redox potentials of VOPO4 and V2O5 observed at around 2.3-2.6 V by using ab-initio calculation are lower than other known technologies, where the potential was observed at around 3V. As a result, lower voltage decreases the energy density of the battery.
Therefore, an object of the present invention is to provide a V2O5 based cathode active material which meets the requirements of a high energy magnesium battery and overcomes the deficiencies of the V2O5 forms conventionally known.
Another object of the present invention is to provide a positive electrode based on the V2O5 based cathode material and a magnesium battery containing the positive electrode having significantly improved energy density and performance in comparison to known magnesium electrochemical devices.